The present invention relates generally to apparatus for safeguarding a computer network against unauthorized entry, and more particularly to circuit apparatus for preventing unauthorized access to a computer coupled to a dial-up telephone line through an automatic-answering modem.
Computer networks vary in complexity from the interconnection of multiple terminals with a host computer to the linking of multiple computers by communications channels. Frequently, the communications channels used for computer networks are ordinary dial-up telephone lines. In a computer network linked by dial-up lines, data are transmitted over analog telephone lines and circuits using analog signaling techniques. Consequently, the computers of such a network are coupled to the dial-up lines through modems (acronym for a modulator/demodulator), which are devices for transforming the digital data signals of the computers to modulated analog signals suitable for transmission over telephone lines and vice versa.
Recently, owing to the increasingly widespread use of computer networks in private business, schools and governmental agencies and the proliferation of terminals and personal computers equipped with communications capability, the problem of protecting computer networks from unauthorized intrusion has been receiving considerable attention. Such unauthorized intrusion may involve the unpermitted use of the computer facilities in the network, the misappropriation of sensitive or proprietary data, the alteration or destruction of valuable data or programs, and, in aggravated cases, the stealing of money, property, services or financial instrument by computer means. The intruder may be an amateur trespasser, commonly referred to as a hacker, who breaks into computers for the challenge and the fun of it or a professional computer criminal motivated by illegal profit.
Computer networks which are linked by dial-up telephone lines are particularly vulnerable to unauthorized entry, owing to the accessibility of the communications channels of such networks by the general public. Normally, unlisted telephone numbers are used for the computer sites in the network. Other conventional techniques for safeguarding dial-up computer networks such as password authentication and data encryption provide protection only after a computer is accessed.
In a typical dial-up network, a telephone line is connected directly to an automatic-answering modem coupled to a computer. When the telephone number of the computer site is dialed by a caller seeking access, a 20 Hz ringing signal appears on the line. This ringing signal causes the modem to "ring trip" (i.e., answer the call) and to send a carrier signal (e.g., a 1070 Hz tone) to the caller. Upon receiving the carrier the caller is required to initiate an exchange of "handshake" signals with the modem within 20 to 30 seconds. Otherwise, the modem disconnects from the line. Normally, the exchange of handshake signals is initiated by the caller activating a local modem.
Once handshake is completed, the caller receives an acknowledgement signal indicating that the computer sought to be accessed is connected to the line. If password authentication is implemented, the computer is programmed to require a valid "password" to be sent by the caller before transactions are permitted. Generally, the computer is also programmed to disconnect from the line if a valid password is not received after a certain number of attempts by the caller in sending such a password. If data encryption is used, the computers of the network are typically provided with encryption/decryption hardware of software such that an unauthorized caller without the proper cryptographic equipment is prevented from using the computers and the data transmitted by the computers.
The problem with the foregoing known techniques for safeguarding a dial-up computer network is that such techniques tend to be ineffective against a determined intruder for the reason that the automatic-answering modem used with a computer of the network brings the computer on line without access control once the telephone number of the computer site is dialed and handshake with the modem is performed. Furthermore, because the modem automatically responds to a line ringing signal by sending a carrier tone, unlisted telephone numbers of computer sites are susceptible to discovery by an intruder using automatic equipment to randomly dial telephone numbers and identifying those numbers that respond with a carrier tone. Once the telephone number of a computer site is known, a caller can obtain virtually unlimited access time to the computer at the site by repeatedly re-dialing that number. Under those conditions, even the most elaborate password scheme is vulnerable to penetration by random or systematic search techniques, particularly since the re-dialing of the computer and the searching for the password can be performed automatically using computerized equipment. Likewise, data encryption methods are also susceptible of being broken by automated deciphering techniques given sufficient access time. Therefore, a need clearly exists for an effective technique for preventing unauthorized access to a computer coupled to a dial-up computer network through an automatic-answering modem.